1. Field of the Invention
The present invention generally relates to the design and fabrication of semiconductor devices. Specifically, the present invention relates to a fuse for use in a semiconductor device, to methods of fabricating the fuse, and to a semiconductor device that includes the fuse. In particular, the present invention relates to a silicide fuse of a semiconductor device and to a semiconductor memory device that includes the silicide fuse. More particularly, the present invention relates to a semiconductor device that includes two diffusion regions disposed substantially within a well of opposite conductivity type, each of which communicates with an end of a metal silicide fuse.
2. Background of Related Art
Computers typically include devices that store data, such as memory devices. A first type of memory device is referred to as a read only memory (“ROM”) device, in which data is permanently stored and cannot be overwritten or otherwise altered. Thus, ROM devices are useful whenever unalterable data or instructions are required. ROM devices are also nonvolatile devices, meaning that the data is not destroyed when power to these devices is shut off. ROM devices are typically programmed during their fabrication by making permanent electrical connections in selected portions of the memory device. One disadvantage of ROM devices is that their programming is permanently determined during fabrication and cannot, therefore, be changed. Thus, when new programming is desired, a ROM device must be newly configured to be wired in accordance with the desired program.
Another type of memory device is a programmable read only memory (“PROM”) device. Unlike ROM devices, PROM devices may be programmed after their fabrication. To render PROM devices programmable, some PROM devices are provided with an electrical connection in the form of a fusible link, which is also typically referred to as a fuse. A considerable number of fuse designs are known and employed in PROM devices. Exemplary fuse designs are disclosed in PROM Fuse Design Scales to Sub0.25 Micron, Electronic Engineering Times, Sep. 29, 1997, p.4, in IEEE Transactions on Electron Devices, Vol. 33, No. 2, p.250-253 (February 1986), and in U.S. Pat. Nos. 5,672,905, 4,679,310, 5,264,725, 4,935,801, 4,670,970, 4,135,295, and 4,647,340.
An exemplary use of fuses in semiconductor devices has been in redundancy technology. Redundancy technology improves the fabrication yield of high-density semiconductor devices, such as static random access memory (“SRAM”) devices and dynamic random access memory (“DRAM”) devices, by facilitating the substitution of a redundant program circuit for a failed program circuit that could otherwise render the semiconductor device useless. The failed circuit may be bypassed and the redundant circuit activated or programmed by selectively programming, or “blowing,”, fuses of the semiconductor device.
Fuses are perhaps the simplest and most compact means of programming a semiconductor memory device with a particular wiring scheme. Perhaps the most common fuse design is a conductive layer, typically comprising metal or polysilicon, which is narrowed or “necked down” in one region. To blow the fuse, a relatively high electrical current, or programming current, is applied to the fuse. The programming current heats the metal or polysilicon of the fuse to a temperature above the melting point of the metal or polysilicon. As the fuse melts, the metal or polysilicon of the fuse “blows” or becomes discontinuous, breaking the conductive link across the fuse. Typically, the fuse becomes discontinuous at the narrowed region since the material volume at the narrowed region is smaller than that of other portions of the fuse and, consequently, the current density is highest and the temperature increases most quickly at the narrowed region of the fuse. By selectively “blowing” the fuses of a PROM device, the PROM device is programmed to have a desired wiring scheme with conductive and substantially nonconductive fuses, thereby imparting each location of the PROM with a corresponding value of “1” or “0” representative of the conductivity state of the fuse (i.e., either conductive or substantially nonconductive), an array of which values comprises the data stored in the semiconductor device.
As an alternative to employing an electrical current to program a semiconductor device, a laser may be employed to blow selected fuses. The use of lasers to “blow” fuses has, however, become increasingly difficult as the size of the features of semiconductor devices, including the fuses thereof, decreases and as the density of features of semiconductor devices increases. Since the diameter of a laser beam should be smaller than the fuse pitch, the utility of laser beams to “blow” fuses begins to diminish with fuse pitches that are about the same or less than the diameters of conventional (e.g., about 5 microns) and state of the art laser beams.
As the programming current or laser beam intensity required to “blow” a conventional fuse may damage regions or structures of the semiconductor device proximate the fuse, conventional fuses are somewhat undesirable. Moreover, if the use of laser beams is desired to program the fuses of a semiconductor device, the fuse pitch and, thus, the density of structures on the semiconductor device may be limited.
When a metal fuse is disposed adjacent a doped silicon or doped polysilicon structure to bridge selected regions thereof, the resistance of the adjacent silicon or polysilicon may not differ significantly from the resistance of the fuse. Thus, upon “blowing” the fuse, the adjacent silicon or polysilicon may continue to transmit current similar to the current carried across an intact fuse. This is especially problematic when such a fuse is disposed adjacent a region, such as an n-well, of a semiconductor substrate conductively doped to have a first conductivity type to bridge two separate conductive wells, such as p-wells, of a second conductivity type, opposite the first conductivity type, disposed adjacent the region of first conductivity type. If the fuse “blows” in a manner that leaves a section of a second, or outlet, side of the fuse that overlaps both a p-well and a portion of the common n-well, current may continue to pass into a p-well from a first side of the “blown” fuse, into the n-well, and out of the n-well to the portion of the second side of the “blown” fuse that overlaps the n-well. Thus, a fuse that blows in such a manner may undesirably conduct current having substantially the same characteristics as current conducted across an intact fuse.
Moreover, since electrically conductive metal silicide structures may be fabricated by annealing metal to an adjacent silicon or polysilicon structure, metal fuses that are disposed adjacent to silicon or polysilicon structures may conduct current even after being “blown.” This may occur if a high enough current is applied to the fuse or if the fuse is otherwise heated to a sufficient temperature to cause the metal of the fuse to anneal to the adjacent semiconductive material and to thereby form a metal silicide that may bridge the discontinuous portion of the fuse. The “blown” fuse may thus undesirably conduct current having substantially the same characteristics as current conducted across an intact fuse.
Accordingly, there is a need for a fuse that may be fabricated adjacent a semiconductive region of a state of the art semiconductor device and that, upon programming, or “blowing,” the fuse has a significantly different resistance than the previously intact fuse. There is also a need for a fuse that can be fabricated by known semiconductor device fabrication techniques.